COKING OF WASTE ALKALINE SULFITE PULP LIQUOR AT LOWERED pH

ABSTRACT

The liquid phase coking of waste alkaline sulfite pulp liquors can be effected at a faster rate and a lower temperatures by lowering the pH of such liquors by at least one pH unit. Also desirably, both the coke yield is increased and the Chemical Oxygen Demand of the resulting effluent is lowered. The pH lowering is achieved by adding sulfur dioxide to the waste liquor either as free SO2 or a water solution of SO2. The effluent and the chemicals recovered after coking can be recycled in the paper pulp process from which they were recovered.

United States Patent Qole et al. Sept. 17, 1974 COKING OF WASTE ALKALINE SULFITE FOREIGN PATENTS OR APPLICATIONS PULP LIQUOR AT LOWERED PH 847,218 1970 Canada [75] Inventors: Edward L. Cole, Fishkill; Howard V.

HesS Glenham; wllham Franz Primary Examiner-S. Leon Bashore Gardmer an of Assistant Examiner-Alfred DAndrea, Jr. [73] Assignee: Texaco Inc., New York, NY. Atmmey, Agent, y; RieS [22] Filed: June 3, 1971 [21] Appl. No.: 149,425 [57] ABSTRACT The liquid phase coking of waste alkaline sulfite pulp liquors can be effected at a faster rate and a lower 52] us. 01 162/31, 162/36, 423/207 temperatms by lowering the pH of such liquors by at [51] IL. Cl D216 11/02, D210 11/14 least one pH unit Also desirably, both the coke yield [58] Field Of Search 210/56, 63, 71, 21; 23/48, i increased and the Chemical Oxygen Demand of the 23/49 2099; 201/25; 162/30 resulting effluent is lowered. The pH lowering is 47; 423/207 208 achieved by adding sulfur dioxide to the waste liquor either as free S0 or a water solution of S0 The ef- [56] References C'ted fluent and the chemicals recovered after coking can UNITED STATES PATENTS be recycled in the paper pulp process from which they 3,272,739 9/1966 Earle et a1. 210/71 X were recovered- 3,558,426 l/l97l Hess et allv 1 .1 162/30 3,649,534 3 1972 Schotte 210/63 Clams 1 Drawmg Flgure LIQUOR EFFE CT OF ADDING SULFUR DIOXIDE COKING 550F IIOO PSEG 1/ 1/ 70 if i ADDED S02 TO GIVE A pH OF 1.9 m E 60 .J o e 1 42$ 50 1 m" 2 -ORIGINAL LIQUOR pH 3g o Lu 1- l-LE o 3 D c O 2 E 0 lo 4o 6O I00 no I20 240 COKING COD REMOVAL FROM WASTE ALKALINE PAIHEI] SEN mm COKING WASTE ALKALINE SULFITE LIQUOR- EFFECT OF ADDING SULFUR DIOXIDE COKING 550F IIOO PSEG ADDED S0 TO GIVE A pH OF L9 I o a:

IO 20 3O 4O 5O 6O 7O 80 90 I00 COKING TIME, MINUTES-- IIO I20 N 240 COKING OF WASTE ALKALINE SULFITE PULP LIQUOR AT LOWERED PH BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved process for treating waste alkaline sulfite pulp process liquor containing organic matter dissolved from wood.

2. Description of the Prior Art In the production of pulp and paper, wood is treated with chemicals to remove lignin binder from the cellulose fibers of the wood. In the process, about one-half the dry weight of the tree processed is dissolved in the pulping liquor and only about half recovered as pulp that is subsequently processed to useful paper products. The soluble portion, most of which is discarded as waste in the sulfite pulping process, presents a serious disposal problem. According to recent reports in the trade literature, sulfite liquor from over 100 pulp mills in the United States and Canada is disposed of by pumping the waste liquor into the nearest river or other body of water. Legislation in many states now prohibits the discharge of sulfite waste liquor into streams, resulting in efforts by mills to find the best method of utilizing or disposing of the waste liquor.

Acid cooking liquor for the sulfite process is usually prepared by the reaction between sulfur dioxide and limestone in the presence of water forming calcium bisulfite Ca(HSO Instead of limestone, which forms a calcium based sulfite acid liquor, the acid cooking liquor may be prepared from ammonium carbonate, ammonium hydroxide, magnesium carbonate, magnesium hydroxide, or the carbonate, bicarbonate, sulfite or hydroxide of sodium.

The reactions which take place in the cooking process involve formation of an ammonium or alkali metal salt of lignin sulfonic acid which is soluble and is dissolved out of cellulose in the cooking process. Sufficient metallic ion (or alternatively the ammonium ion) to form the salt of lignin sulfonic acid must be present in the cooking process to produce high yield of pulp.

Heretofore, however, it has not been possible with sulfite pulps to obtain strength and other desirable properties equal to those of sulfate or kraft pulps produced by cooking pulp in a solution of sodium hydroxide and sodium sulfide. Important objections to the kraft process has always been the complicated recovery system and the offensive odors released owing to the production of mercaptans.

Lately, a new process has been found which compares favorably with the kraft process and operates in a sulfide-free alkaline sulfite liquor in which the alkalinity is maintained throughout the cook at about between pH 8 and pH I l as measured in situ at the cooking temperature, the pH being maintained within these limits by the addition of a strong base. This process is described in Canadian patent No. 847,218 issued on July 21, 1970 to O. V. Ingruber et al. and assigned to Canadian International Paper Company. As pointed out in the Aug. 24, 1970 edition of Chemical Week at page 31, it does not appear that this process includes an adequate chemicals recovery system. A possible reason for this lack may be the strong basic character of the waste sulfite liquor. Indeed attempts to treat this waste liquor by coking according to the method of commonly assigned U.S. Pat. No. 3,507,788 indicated that the rate of coke formation was slow.

SUMMARY OF THE INVENTION Unexpectedly it has been discovered that lowering the pH of the waste alkaline sulfite liquors facilitates liquid phase coking of these liquors thereby providing an improved means of removing from the liquors dissolved lignins with an attendant recovery of chemicals. Lowering of pH is achieved by adding sulfur dioxide to the waste alkaline sulfite liquor to give at least one pH unit reduction.

DESCRIPTION OF SPECIFIC EMBODIMENTS It has been found that any addition of sulfur dioxide to bring the waste alkaline sulfite liquor to a less alkaline condition prior to coking in the absence of air is beneficial. Thus if the pH of the liquor is from pH 8 to about pH II a pH reduction from pH 8 to pH 7 or pH II to pH 10 is advantageous insofar as coking and recovery of chemicals is concerned. However, the greater the reduction in pH, the greater the response of the waste to coking. Generally it is preferred to reduce the pH by at least one unit, e.g., 8 to 7 but is is better to reduce the pH of the waste alkaline sulfite liquor to within the range of pH 7 to pH 3.

The operating temperatures during the coking step will range from about 400-700F but preferably will be 450-650F.

The operating pressure is the autogeneous pressure of about 300 -3,500 psig.

The coking time can vary from 0.5 minutes to two hours. Following coking the coke is separated by filtrations and the coke burned to provide process heat and a source of sulfur dioxide. Any gases formed are vented and preferably burned to provide an additional source of sulfur dioxide for recycle. The filtrate is recycled after being refortified and chemically adjusted to provide the alkaline sulfite pumping liquor.

The process of the invention can be carried out in apparatus similar to that described and shown in commonly assigned U.S. Pat. No. 3,507,788 except that the use of an oxidizer unit for oxidizing the effluent is not necessary. The apparatus can then consist mainly of a tubular heater of the type commonly used in the processing of petroleum wherein the temperature of the waste liquor is raised to the desired coking temperature at a pressure sufficient to prevent the vaporization of water. The hot waste liquor can then be introduced into a separator suitable for holding the heated waste liquor at the desired processing temperature and pressure. Gas released during coking is discharged from the separator through a relief valve. Coke settles from the aqueous liquid and collects in the lower part of the separator from which it is discharged through a valve into a flow tank. Aqueous effluent is withdrawn from the upper part of the separator and is reused in the pulping process. Where otherwise convenient, such as when the quantities of liquor are small, autoclaves may be used.

As shown in the accompanying graph, adding to the waste liquor to lower its pH prior to coking, drastically reduces the Chemical Oxygen Demand (COD) of the coked effluent relative to a basic waste liquor for the same temperature, pressure and coking time. Thus coking a liquor having a pH of 1.9 for 1 10 minutes reduced its COD by 80 percent while coking another sample having a pH of 12 at the same temperature and pressure for the same time only reduced the COD by 58 percent. This difference is all the more important since huge volumes of waste liquor are generally treated in paper making plants.

That coking is facilitated by the practice of the present invention was evidenced by the observation that lignins could be coked at a lower temperature. In addition, a light straw-colored liquid recycle was obtained when the pH of the liquor was reduced whereas a dark colored recycle liquor was obtained when the alkaline liquor was coked in the basic state.

It will be appreciated that the present invention is applicable to all alkaline sulfite'containing liquid wastes quor, pl-ll2 of Example 1. These are presented as Example 11 through V in Table 1, below. In all the runs coking was done at 550F with coking times varying from five minutes to 120 minutes. It will be noted that as the result of coking minutes, 0.42 parts by weight of dry coke was recovered and the filtrate had the high COD value of 101,429 mgO /l relative to the charge COD of 146,734 mgO /l. Coking for successively longer times resulted in the separation of increasing amounts of coke with an attendant decrease in COD, i.e., from a value 101,429 after 1 minute to 56,918 after 2 hours.

TABLE 1 COKINO ALKALINE SULFITE LIQUOR FROM EXAMPLE I EQUIPMENT:

Stainless Steel Bombs High Tin Metal Bath Ceramic Buchner Funnel PROCEDURE LIQUOR IN BOMBS COKED IN METAL BATH, COOLED AND FILTERED. EXAMPLE II III IV V SAMPLE A B C D CHARGE, Parts bywt' 60 60 60 60 PROCESS CONDITIONS:

TEMPERATURE, F 550 550 550 550 PRESSURE, PSIG H 1125 1125 1125 TIME AT TEMP, MIN 5 1O 20 I20 FILTER TIME, SEC 10 38 36 PRODUCTS:

FILTRATE, parts by vol 585 57.8 55.7 48.9 FILTER CAKE, WET, parts by wt 0.9 1.9 3.5 8.2 FILTER CAKE, DRY, parts by wt 0.42 0.42 1.25 2.34 WT SULFUR 1.85 2.00 WT 74 SODIUM 6.3 6.2

SAMPLE FILTRATE FILTRATE FILTRATE FILTRATE SAMPLE A B C D COD, mgo /l 101,429 108,040 92,412 56,918 TOC, 7: I 4.4 4.1 4.0 2.2 COLOR BLACK BLACK BLACK BLACK 100 ml at -80F weighs 110 grams.

regardless of their origin and is not limited to those obtained as a result of practicing the process of Canadian Pat. No. 847,218.

The examples given below are presented to illustrate more clearly the nature of the present invention. It should be noted, however, that the invention is not limited to the embodiments given in the examples.

EXAMPLE I (Analysis of Feed Waste Liquor) The analyses on the alkaline sulfite liquor are as follows:

Sample No. 70-4032 Sulfur, wt 1.40 Sodium, wt 70 3.37 Calcium, wt 7: 0.007

- Carbon, wt 1: 5.4

Nitrogen, wt 0.0001 COD, mg0,,/l 146,734 Na,SO,-, 4.9 pH 12 EXAMPLES Il V EXAMPLES VI VIII (Coking of sulfite liquor with pH lowered by adding 2) Examples VI-VIII as summarized in Table 11 below, show the comparative effect of adding S0 to the alkaline sulfite liquor.

Thus in Example V1, 5.5 parts by weight of S0 was added to 54.5 parts by weight of the alkaline sulfite liquor to change the pH from 12 to 3. This addition represented about a 50 percent excess over that required to neutralize the excess base in the waste liquor. The liquor was then coked at 550F for 5 minutes and the product cooled. The cooled product was filtered to give 3.0 parts of dry coke and a filtrate having a COD of 43,654. This result is to be compared directly with the results of Example 11. Thus by adding S0 3.0-0.42/0.42 X 100 61 2'percent more coke was obtained and the COD was reduced to 43,654 mgO L compared to a COD of 101,429 mgO /l without the use of These very substantial advantages obtained in employing S0 were accompanied by an improved filtrate color, that is, when coking was done without the use of S0 the color was black whereas when S0: was added prior to coking the filtrate color was light yellow.

Examples V and VII provide another direct comparison of the effectiveness of employing added S0 in the coking of the alkaline sulfite liquor. No S0 was added in Example V while in Example VII 5.5 parts of S0 was added to 54.5 parts of liquor thereby giving about 50 percent over the amount needed to neutralize the free base in the waste liquor. Coking was done in both runs at 550F for 2 hours.

TABLE II (Coking Alkaline Sulfite Liquor with Added Sulfur Dioxide) EQUIPMENT:

Stainless Steel Bombs High Tin Metal Bath Ceramic Buchner Funnel PROCEDURE: LIQUOR IN BOMBS COKED IN METAL BATH, COOLED AND FILTERED. EXAMPLE VI VII VIII CHARGE iquor, parts by wt 54.5 54.5 56.5 50, do. 5.5 5.5 3.5 Analysis Free 2. wt 7: 3.1 3.1

NaHSO,, do. 16.2 16.2 11.1

COD, mgO /l 150,360 PROCESS CONDITIONS emperature, 550 550 550 Pressure, psig I250 1250 1150 Time at Temp min 120 5 Filter Time, sec 36 32 Foamed PRODUCTS Filtrate, ml 48.4 42 25.4 Filter Cake, Wet, parts by wt 5.4 5.8 3.4 Filter Cake, Dry do, 3.0 2.8 2.9 Sample Filtrate Coke Filtrate Coke Filtrate Coke Sample No. 18681 186B1C 186B2 186B2C 188CA 188CC Sodium, Wt 7: 1.36 1.79 6.6 COD, mg0,/l 43.654 21,877 48,595 pH 3 3 8 NaHSO wt 7: 6.4 2.1 Color Lt.Yellow Lt.Yellow Black Sulfur, wt 7: TOC. 7:

'Loss due to foaming.

Without the use of added S0 2.34 parts by weight of dry coke was secured and the COD was reduced to 56,918 mgO /l whereas with added S0 2.8 parts by weight of dry coke was recovered and the COD was reduced to 21,877 mgO /1. Coking in the presence of added S0 resulted in a light yellow filtrate whereas in the absence of S0 the filtrate from the coking step was blank.

Example VIII was similar to Example V1 with the exception that less S0 was added to the waste alkaline sulfite liquor. That is, 3.5 parts by weight of S0 was added to 56.5 parts by weight of waste liquor, or about 5.6 percent less than is theoretically required to neutralize the excess NaOI-I present. Coking at 550F for 5 minutes resulted in 2.9 parts by weight of dry coke being recovered, and a dark filtrate having a COD of 48,595 mgO /l. The pH of 8 of the filtrate reflects the lower amount of S0 charged. The experiment teaches that even though less S0 was added to the waste liquor than that required for neutralizing the caustic, coking was made more effective than when S0 was not added, however, it was less effective than when a surplus over that required to neutralize the base was used. Additionally, a lower concentration of sodium in the coke was found when operating with a large surplus of S0 These facts are set forth as follows:

Example Coking Alkaline Sulfite Liquor with Added Sulfur Dioxide EQUIPMENT: Stainless Steel Bombs High Tin Metal Bath Ceramic Buchner Funnel PROCEDURE: Li uor in Bombs was coked in a metal bath, cooled, filtered an% the filter cake washed. Example: IX X1 Charge Liquor, parts by weight 55 SO,, do. 5 5 Anal sis pH L--5l93A L-70-l94A 4 COD, mg 0 /1 153,770 156,705 TOC 5.0 5.6 Sulfur, wt 7: 2.1 3.3

TABLE Ill-Continued Coking Alkaline Sulfite Liquor with Added Sulfur Dioxide EQUIPMENT: Stainless Steel Bombs High Tin Metal Bath Ceramic Buchner Funnel PROCEDURE: Liquor in Bombs was coked in a metal bath. cooled. filtered and the filter cake washed.

Example: IX Xl Process Conditions Temp. F 500 550 550 Pressure. psig 650 1100 1100 Time at Temp. min 5 5 Filter Time. sec 39 46 42 Products Filtrate. parts by vol 42 48 49 Filter Cake. Wet. parts by weight 5.2 5.0

Wash H O. parts by vol 10 10 Wash Filtrate. pans by vol 12 10 1 1 Filter Cake. Dry. parts by weight 1.7 1.7 2.2 Sample: Filtrate Cake Filtrate Cake Filtrate Cake 1938A 1938C 193 CA 193CC 1948A 1948C Sodium. wt 3.49 1,72 3.36 E 2.18 3.61 1.55 COD, MgOQ/l 56,748 48,947 50,628

TOC, wt 2.2 1.9 1.8

EXAMPLES IX-XI EFFECT OF VARYING AMOUNTS OF ADDED S0 Examples IXXl as summarized in Table 111 show additional data on the effect of adding S0 to the alkaline sulfite liquor. These examples differ from the earlier examples as follows:

a. in Example 1X, S0 was added to the sulfite liquor to give a feed having a pH of 5, the coking was done at 500F and the filter cake was washed with water to remove sodium.

b. 1n Example X, the procedure in Example [X was repeated excepting coking was done at 550F. It will be noted that a filtrate having a lower COD was obtained as the result of coking at 550F, i.e., 48,947 MgO /l vs 56,748 mgO /l secured at 500F.

c. in Example X1, S0 was added to the sulfite liquor to give a feed having a pH of 4, coking was done at 550F and the coke was washed to remove sodium.

EXAMPLE Xll FIG. 1 shows in graph form the dramatic effect of adding sulfur dioxide to the waste alkaline sulfite liquor on its response to coking with subsequent removal of coked lignins. Thus upon coking an original waste 1iquor for minutes about 30 percent reduction in COD was achieved, whereas with the same liquor made acid, pH 1.9, with sulfur dioxide the COD reduction was about 71 percent.

To avoid gel formation it is preferred to heat rapidly to the coking temperature at least in the range of 350 to 500F. As described and claimed in our copending application Ser. No. 149,673, now abandoned, filed even day herewith, this heating rate can be about 1 10F to F per minute in the temperature interval of about 350F to 550F.

We claim:

1. In a process for treating alkaline sulfite waste pulping liquors having a pH in the range of pH 11 to pH 8 by coking said liquors in the liquid phase at temperature ranges from about 400 to about 700F. at a pressure of about 300 to 3,500 psig for a coking time in the range of about 0.5 minutes to about 2 hours to form coke, exhaust gases and an aqueous effluent; separating said coke from said effluent; and recovering said effluent to form additional pulping liquor, the improvement consisting of lowering the pH of said liquors by at least one unit by the addition of extraneous S0 before coking same, and burning said coke and said exhaust gases to recover chemical values therefrom for recycling.

2. The process according to claim 1, wherein an excess of S0 is added over and above that required to neutralize the caustic present in said liquors.

3. The process according to claim 1, wherein coking is carried out at between 450 to 650F.

4. The process according to claim 1, wherein the pH of the liquor is reduced to within the range of pH 7 to pH 3 prior to coking.

5. The process of claim 1, wherein the pH of the liquor is reduced to 1.9 before coking. 

2. The process according to claim 1, wherein an excess of SO2 is added over and above that required to neutralize the caustic present in said liquors.
 3. The process according to claim 1, wherein coking is carried out at between 450* to 650*F.
 4. The process according to claim 1, wherein the pH of the liquor is reduced to within the range of pH 7 to pH 3 prior to coking.
 5. The process of claim 1, wherein the pH of the liquor is reduced to 1.9 before coking. 